1. Field of the Invention:
The present invention relates to a thin film transistor (hereinafter referred to as TFT) for a liquid crystal display apparatus and to a method of manufacturing the TFT. More particularly, the present invention relates to a TFT in which a shape of a cross-section of the electrode line pattern is etched to be a tapered shape and to a method of forming an electrode line in which an etching rate is varied to a direction of film thickness by varying a film quality to a direction of film thickness in forming an electrode line.
2. Description of the Background:
FIGS. 5(a) through 5(d) are sectional drawings showing composition of a conventional TFT provided in a conventional TFT liquid crystal display. In FIGS. 5(a) through 5(d) a reference numeral 1 denotes a substrate, numeral 2 denotes a first electrode line, numeral 3 denotes an insulating film, numeral 4 denotes an amorphous silicon (hereinafter referred to as a-Si) semiconductor film, numeral 5 denotes n+ type a-Si semiconductor film, numeral 6 denotes a contact hole for forming a channel, numeral 7 denotes a second electrode line, numeral 8 denotes a passivation film, numeral 9 denotes a resist (photo resist). Further, the first electrode line is to be a gate electrode and the second electrode line is to be a source electrode and a drain electrode.
TFT liquid crystal display comprises a thin film transistor array substrate (hereinafter referred to as "TFT array substrate") on which TFTs normally composed of a semiconductor thin film (hereinafter referred to as semiconductor film) are provided, an opposite substrate arranged to be opposite to the TFT array substrate and a liquid crystal material interposed between the TFT array substrate and the oposite substrate.
So as to display various picture image as a display apparatus, the central area of the TFT array substrate is assigned to be a display area and in the display the TFTs and pixel electrodes are provided in a matrix manner to each pixel so as to apply a voltage to each pixel which is a minimum unit of picture image. Signal lines such as gate lines, source lines and so on are provided between pixels in such a manner that gate lines are provided in parallel to each other and source lines are provided in parallel to each other.
Further, an alignment film and a storage capacitance if necessary are provided on the TFT array substrate. An opposite electrode, color filters, black matrices and so on are provided on the opposite substrate.
The liquid crystal display (hereinafter referred to as LCD) in which such a TFT array substrate is employed is referred to a TFT-LCD.
A method of manufacturing the TFT including each composition is explained below.
First, a first electrode line 2 is formed on a substrate 1 composed of glass and so on as an insulating substrate. The first electrode line 2 is deposited in such a manner in which a metal material composed of for example chrome and so on is deposited by a sputtering method so that the film quality is uniform. Thereafter, a patterned resist (photo resist) 9 is formed; a chemical etchant composed of ammonium cerium nitrate and nitric acid is sprayed on the film; and the first electrode line 2 is formed by etching, so that an end portion of the first electrode line 2 is tapered by subjecting to errosion operation by the chemical etchant. At this moment, a tapered surface is a slant surface crossing to a perpendicular line of the surface of the substrate 1 with a certain range of degrees. The certain range of degrees, i.e., a taper angle exist approximately from 10 degrees to 45 degrees. Therefore, a shape of the cross-section of the first electrode line 2 is a shape in which a thickness (in a direction perpendicular to a surface of the first electrode line) decreases in a direction toward end face 18 (shown in FIG. 1(a)) of the first electrode line so that a length L.sub.2 of a bottom portion of the first electrode line is shorter than a length L.sub.1 of a top portion of the first electrode line (L.sub.2 &gt;L.sub.1). Vicissitudes of the shapes of the TFT are shown in FIG. 5(a) through 5(c).
Thereafter, the insulating film 3 composed of silicon nitride and so on is formed and the TFT array substrate in process is cleaned chemically or physically. When cleaning physically, the TFT array substrate in process is cleaned for example by a scrubber. Next, a semiconductor layer, i.e., amorphous silicon (a-Si) semiconductor film 4 and another semiconductor layer, i.e., n+ type a-Si semiconductor film 5 is formed on the insulating film 3; a contact hole 6 is provided; and a second electrode line 7 is formed in the similar manner as the method in which the first electrode line 2 is formed. Finally, a passivation film 8 is formed to be TFT structure shown in FIG. 5(d).
Among the above-mentioned method for manufacturing the TFT, when forming metal materials for the first eletrode line 2 and the second electrode line 7 by a sputtering method, a plurality of targets can be employed. The compositions of the targets are different from each other. The target is shaped into like a plate so that a material which is to be deposited on a substrate and so on is attacked by argon gas to deposit the material. So as to deposit a film the composition of which is different gradually, such targets the composition of which are different from each other are employed. The word "gradually" means that a plurality of depositing steps are performed successively without taking out semiconductor products in process from the depositing apparatus.
In order to manufacture the liquid crystal display apparatus in which there is employed such a TFT array substrate that the TFTs, gate lines, source lines and other common lines are provided in matrix manner on a glass substrate to assign a display area, and input terminals, auxiliary lines, TFT drive circuits and so on on a periphery of the display area. At this moment, electric conductive thin films (hereinafter, referred to as "conductive film") and insulating thin film (hereinafter, referred to as "insulating film") if necessary are provided so as to obtain the functions respectively. An opposite electrodes are provided on the opposite substrate. Further, color filters and black matrices are also provided on the opposite substrate.
Liquid crystal display apparatus is manufactured in such a manner that the TFT array substrate and the opposite substrate is manufactured; the TFT array substrate and the opposite substrate are adhered on the periphery of the TFT array substrate and the periphery of the opposite substrate with the distance between the TFT array substrate and the opposite substrate being kept a desired gap so that a liquid crystal material is injected between the TFT array substrate and the opposite substrate.
In the method described above manufacturing the TFT, processing condition data of etching step is controlled so as to control the taper angle of the electrode line such as the first electrode line, the second electrode line and so on (hereinafter, referred to as "electrode line" for a general term), within a desired range of angle.
That is to say, since the taper angle cannot be controlled within the desired range of angle if impurities caused by etching are accumulated in chemical etchant, it is required either such a manner that the chemical etchant is exchanged to new or such a manner that a plurality of targets are prepared, allowing a frequency of exchanging to be increased, the used chemical etchant composed of ammonium cerium nitrate and nitric acid must be exchanged for fresh chemical etchant frequently (one time or two times a day) so as to keep impurities concentration in the chemical etchant below a given amount.
There arises a problem that checking of exchanging such chemical etchant and impurities concentration requires very large production cost in the etching step. Further, there arises another problem that a number of targets employed in deposition step cannot be reduced.